The present invention generally relates to an electric motor and more particularly, to an electric motor the output shaft of which is constructed to perform a reciprocating rectilinear motion.
Normally, an electric motor has the output shaft constructed so as to be rotated. However, in recent years, there has been required an electric motor having an output shaft which performs a rectilinear motion for use in an automatic control means, particularly a rectilinear displacement in a stepless manner. Conventionally, in order to meet the requirement as described above, a rotary electric motor has been employed, with external gears and the like being provided to convert the rotary motion into the rectilinear motion, thus making the mechanism complicated and the motor itself large.
In order to overcome the disadvantages as described above, there has heretofore been proposed by the present inventors an improved electric motor of the above described type, for example, in Japanese Utility Model Application Jitsugansho No. 52-159555 (Japanese Laid Open Utility Model Publication Jikkaisho No. 54-85305), in which rotational movement of the rotor is converted inside the motor into rectilinear motion for transmission to the output shaft. By the above arrangement, the mechanism required for the rectilinear motion is simplified, with consequent reduction in the size of the electric motor.
However, the above proposed electric motor achieved forward and backward movements of the output shaft by bringing stops provided on the output shaft into contact with a threaded driving member, and had the disadvantage that, since the regulation of the movements of the output shaft was effected through face contact between stops and the threaded driving member, biting or cutting-in due to abnormally excessive contact force tended to occur between the output shaft and the threaded driving member, thus making it difficult to effect the subsequent starting of the rotor.
Meanwhile, in recent years, a higher output is has been required for an electric motor of the above-described type including a greater allowable load and greater durability along with the higher output.